Melatonin (N-acetyl-5-methoxytryptamine) is a natural product isolated from the pineal glands of beef cattle. It is reported by Ying and Greep, Endocrinology, 92 (1), 333-5 (1973), that melatonin has an inhibitory effect on ovulation in cyclic rodents. This inhibitory effect is realized when melatonin is administered intracardially in relatively large doses during the critical period of proestrus. However, when melatonin is administered orally, its inhibitory effect on ovulation is substantially diminished over that available by intracardial administration. It now has been discovered that a particular class of tryptamines, N-acyl-5-methoxy-6-halotryptamines, exhibit a high level of activity as ovulation inhibitor even when administered orally.